Chemical looping combustion or CLC: in the text hereafter, what is referred to as CLC (Chemical Looping Combustion) is an oxidation-reduction or redox looping method on an active mass. It can be noted that, in general, the terms oxidation and reduction are used in connection with the respectively oxidized or reduced state of the active mass. The oxidation reactor is the reactor where the redox mass is oxidized and the reduction reactor is the reactor where the redox mass is reduced.
Within a context of increasing world energy demand, capture of carbon dioxide (CO2) for sequestration thereof has become an indispensable means to limit greenhouse gas emissions harmful to the environment. The Chemical Looping Combustion (CLC) process allows to produce energy from hydrocarbon-containing fuels while facilitating capture of the CO2 emitted upon combustion.
CLC consists in implementing redox reactions of an active mass, typically a metal oxide, for splitting the combustion reaction into two successive reactions. A first oxidation reaction of the active mass, with air or a gas acting as the oxidizer, allows the active mass to be oxidized. This reaction is highly exothermic and it generally develops more energy than the combustion of the feed. A second reduction reaction of the active mass thus oxidized, by means of a reducing gas, then allows to obtain a reusable active mass and a gas mixture essentially comprising CO2 and water, or even syngas containing hydrogen (H2) and nitrogen monoxide (CO). This reaction is generally endothermic. This technique thus enables to isolate the CO2 or the syngas in a gas mixture practically free of oxygen and nitrogen.
The balance of the chemical looping combustion, i.e. of the two previous reactions, is globally exothermic and it corresponds to the heating value of the treated feed. It is possible to produce energy from this process, in form of vapour or electricity, by arranging exchange surfaces in the active mass circulation loop or on the gaseous effluents downstream from the combustion or oxidation reactions.
U.S. Pat. No. 5,447,024 describes for example a chemical looping combustion method comprising a first reactor for reduction of an active mass using a reducing gas and a second oxidation reactor allowing to restore the active mass in its oxidized state through an oxidation reaction with wet air. The circulating fluidized bed technology is used to enable continuous change of the active mass from the oxidized state to the reduced state thereof.
The active mass going alternately from the oxidized form to the reduced form thereof, and conversely, follows a redox cycle.
Thus, in the reduction reactor, active mass (MxOy) is first reduced to the state MxOy-2n-m/2 by means of a hydrocarbon CnHm that is correlatively oxidized to CO2 and H2O, according to reaction (1), or optionally to a mixture CO+H2, depending on the proportions used.CnHm+MxOy→nCO2+m/2H2O+MxOy-2n-m/2  (1)
In the oxidation reactor, the active mass is restored to its oxidized state (MxOy) on contact with air according to reaction (2), prior to returning to the first reactor.MxOy-2n-m/2+(n+m/4)O2→MxOy  (2)
In the above equations, M represents a metal.
The efficiency of the circulating fluidized bed chemical looping combustion (CLC) method is based to a large extent on the physico-chemical properties of the redox active mass.
The reactivity of the redox pair(s) involved and the associated oxygen transfer capacity are parameters that influence the dimensioning of the reactors and the rates of circulation of the particles. The life of the particles depends on the mechanical strength of the particles and on the chemical stability thereof.
In order to obtain particles usable for this process, the particles involved generally consist of a redox pair selected from among CuO/Cu, Cu2O/Cu, NiO/Ni, Fe2O3/Fe3O4, FeO/Fe, Fe3O4/FeO, MnO2/Mn2O3, Mn2O3/Mn3O4, Mn3O4/MnO, MnO/Mn, Co3O4/CoO, CoO/Co, or of a combination of some of these redox pairs, and sometimes of a binder providing the required physico-chemical stability.
The NiO/Ni pair is often mentioned as the reference active mass for the CLC process due to its oxygen transport capacities and its fast reduction kinetics, notably in the presence of methane, despite the high toxicity of nickel oxide (it is classified as a CMR1 substance: Carcinogenic, Mutagenic or toxic for Reproduction of class 1), leading notably to significant constraints on the fumes filtration system, and despite its high cost. Indeed, since nickel oxide does not occur in the natural state with a sufficient concentration to allow interesting properties for the CLC process to be obtained, it is generally used concentrated in synthetic active mass particles whose manufacturing cost is high.
More generally, a major issue raised by the implementation of a CLC process is the cost of the active mass. Since the CLC process requires circulation of the solid in reactors where gas velocities are relatively high, continuous consumption of the solid through attrition cannot be prevented. If the cost of the active mass is relatively high, the make-up active mass item can become a significant part of the operating cost. This is particularly the case for synthetic particles whose manufacturing cost is high.
It is therefore important to find an inexpensive active mass in order to reduce the impact of the cost of the particles on the price of CO2 capture through CLC.
Besides, the use of natural ores as active mass for the CLC process, such as ilmenite or manganese ores, which can provide a satisfactory solution in terms of cost, is known.
However, the use of such ores is generally less suited for combustion of gas feeds such as methane than for the combustion of solid or liquid feeds, in terms of process performance.
There is therefore a need for an efficient CLC process, notably in terms of feed conversion, suited to the treatment of a gaseous hydrocarbon feed and that can use an inexpensive material as the redox active mass, complying with environmental standards in terms of toxicity and reducing emissions.